1. Field of the Invention
The present invention relates to a door inner panel as automobile parts and a method of manufacturing the same.
2. Description of the Related Art
Door inner panels using steel sheets having different thicknesses (tailor welded blank) are used for vehicle side doors or vehicle back doors of automobiles. The steel sheets having different thicknesses (which are called different thickness steel sheets) are formed using different thickness connection technology by means of butt welding.
The door inner panel as opening/closing door for an automobile is composed of the tailor welded blank (which is a member or parts after joining different thickness steel sheets) in which a thick sheet is provided in a desired region on a hinge side of each door inner panel such that a door mounting portion (a hinge portion) with which each door inner panel is mounted to a vehicle body has enough hinge rigidity.
In the door inner panels, the thick sheet is preferably provided in a smallest region for the purpose of weight saving within a range in which the rigidity of the hinge portion is ensured. However, a weld line in the different thickness connection is fixed in position in the door inner panels in view of panel formability. The thick sheet is thus provided not only in a minimum required region, but also in an unnecessary region. Thus, the door inner panels increase in weight.
To solve such inconvenience, a method of manufacturing a door inner panel, in which a plurality of panels, at least three steel sheets, with different sheet thicknesses are linearly welded together to constitute a different thickness steel sheet so as to enable weight saving, has been proposed (Patent Document 1: Japanese Patent Laid-Open Publication No. 2011-143892).
In the conventional door inner panel obtained by the different thickness connection technology in which at least three steel sheets (panels) are butted and linearly welded together, butting accuracy is deteriorated by thermal strain at an intersection between weld lines or a corner portion thereof at the time of butt welding between the panels. Thus, the weld joining cannot be performed at high accuracy.
In addition, since the intersection or the corner portion is subjected to heat input twice, a welding failure tends to occur. Moreover, in the door inner panel obtained by the butt welding of at least three steel sheets, stress concentration occurs in the intersection between weld lines or the corner portion thereof due to its shape. Thus, there also occurs a defect such that a crack is generated in press forming after the butt welding.
As described above, the door inner panel using the different thickness steel sheets, into tailor welded blank, formed by the linear butt welding of at least three steel sheets (panels) in conventional technology provides defects or inconveniences such that a welding failure tends to occur since the intersection between weld lines or the corner portion thereof is subjected to heat input twice, and that it is difficult to perform the press forming due to the weld shape in which stress concentration occurs.